UNITED NATIONS (AP) — Russia’s U.N. ambassador said Tuesday that Russian experts determined that Syrian rebels made sarin nerve gas and used it in a deadly chemical weapon attack outside Aleppo in March.

Ambassador Vitaly Churkin blamed opposition fighters for the March 19 attack in the government-controlled Aleppo suburb of Khan al-Assal, which he said killed 26 people, including 16 military personnel, and injured 86 others.

The rebels have blamed the government for the attack. The U.S. Britain and France have said they have seen no evidence to indicate that the opposition has acquired or used chemical weapons.

In Washington, White House spokesman Jay Carney said “We have yet to see any evidence that backs up the assertion that anybody besides the Syrian government has had the ability to use chemical weapons or has used chemical weapons.”

Churkin told reporters after delivering an 80-page report to Secretary-General Ban Ki-moon that the Assad regime asked Russia, its closest ally, to investigate the attack after a U.N. team of chemical weapons experts was unable to enter the country in a dispute over the probe’s scope.

Acting U.S. Ambassador Rosemary DiCarlo said Syrian President Bashar Assad should now allow U.N. chemical weapons experts into the country to conduct an investigation of the Khan al-Assal incident as well as other allegations of chemical weapons use by the U.S., U.K., and France.

The samples taken from the impact site of the gas-laden projectile were analyzed at a Russian laboratory certified by the Organization for the Prohibition of Chemical Weapons, Churkin said.

He said the analysis showed that the unguided Basha’ir-3 rocket that hit Khan al-Assal was not a military-standard chemical weapon.

Churkin said the results indicate it “was not industrially manufactured and was filled with sarin.” He said the samples indicated the sarin and the projectile were produced in makeshift “cottage industry” conditions, and the projectile “is not a standard one for chemical use.”

The absence of chemical stabilizers, which are needed for long-term storage and later use, indicated its “possibly recent production,” Churkin said.

“Therefore, there is every reason to believe that it was the armed opposition fighters who used the chemical weapons in Khan al-Assal,” Churkin said.

“According to information at our disposal,” he added, “the production of ‘Basha’ir 3′ unguided projectiles was started in February 2013 by the so-called ‘Basha’ir al-Nasr’ brigade affiliated with the Free Syrian Army.”

On Monday, Syria invited Ake Sellstrom, head of the U.N. fact-finding mission on allegations of chemical weapons use in Syria, and U.N. disarmament chief Angela Kane to visit Damascus for foreign-minister level talks on conducting a probe of just the Khan al-Assal attack.

The Russian ambassador strongly backed the idea, calling it “a promising process” that hopefully will lead to an investigation.

Britain, France and the United States have provided the secretary-general with information on other alleged chemical weapons attacks in Syria. Ban has repeatedly said he wants a broader investigation than just Khan al-Assal.

“We support a thorough investigation of all credible allegations,” Churkin said, but added that Russian experts “were not impressed at all” by the material provided to them by the U.K., U.S. and France.

At the White House, Carney said that Syria’s President “Bashar al-Assad called for a U.N. investigation into the use of chemical weapons and then he blocked the ability of the United Nations to conduct that investigation. The way to answer this question is to allow the United Nations to investigate.”

President Barack Obama’s administration says it has “high confidence” that Syrian President Bashar Assad’s forces have killed up to 150 people with sarin gas.

In a letter to the secretary-general on June 14, then-U.S. Ambassador Susan Rice said the U.S. had determined that sarin was used in the March 19 attack on Khan al-Assal and also in an April 13 attack on the Aleppo neighborhood of Shaykh Maqsud. She said unspecified chemicals, possibly including chemical warfare agents, were used May 14 in an attack on Qasr Abu Samrah and in a May 23 attack on Adra.

The use of a chemical weapon crossed Obama’s “red line” for escalating U.S. involvement in the conflict and prompted the decision to send arms and ammunition to the opposition, not just humanitarian aid and non-lethal material like armored vests and night goggles.

Churkin said Russia plans to provide the 80-page report to the U.S., U.K. and France, and “I hope they find it persuasive.” But he said it will not be made public.

U.N. spokesman Martin Nesirky had no immediate comment on the issue, noting that the Russian ambassador had delivered the “weighty and quite technical” report only minutes earlier. He said the Department of Disarmament Affairs would study it and provide guidance to the secretary-general.